feat: 🎸 Refactor serializer and model for managed-group

Refactor serializer and model with suggestions from previous PR for
managed-group.

addons/api/addon/models/managed-group.js

@@ -1,12 +1,21 @@
 import GeneratedManagedGroupModel from '../generated/models/managed-group';
 import { attr } from '@ember-data/model';
 import { fragment } from 'ember-data-model-fragments/attributes';
+import { A } from '@ember/array';
 
 export default class ManagedGroupModel extends GeneratedManagedGroupModel {
-  // =error attributes
-  @attr('string', { readOnly: true }) filter;
-
   // =attributes
+  /**
+   * Members is read-only under normal circumstances.  But members can
+   * be persisted via calls to `addMembers()` or `removeMembers()`.
+   */
+  @attr({
+    readOnly: true,
+    defaultValue: () => A(),
+    emptyArrayIfMissing: true,
+  })
+  member_ids;
+
   @fragment('fragment-managed-group-attributes', { defaultValue: {} })
   attributes;
 }